HIV and its monkey cousin SIV can carry the alpha-4 beta-7 integrin receptor in their outer envelope, which helps the virus enter gut cells during early infection, according to research presented at the recent Conference on Retroviruses and Opportunistic Infections (CROI) in Seattle. This finding may help explain how an antibody against alpha-4 beta-7 produced sustained viral remission in monkeys.

In a report greeted with much excitement last fall, Anthony Fauci from the National Institutes of Allergy and Infectious Diseases(NIAID) and colleagues announced that they had induced sustained remission of SIV in macaque monkeys treated with antiretroviral therapy (ART) plus antibody therapy. Treated monkeys not only had undetectable viral load for up to 2 years after stopping ART, but they also showed replenishment of key immune cells in the gut.

The researchers used a "primatized" version of a monoclonal antibody against the alpha-4 beta-7 integrin that is similar to the drug vedolizumab (Entyvio), which is approved for treatment of inflammatory bowel disease.

At the time of the report, the researchers said they did not know how the alpha-4 beta-7 antibody kept the virus in check.

At CROI Christina Guzzo from NIAID presented results from laboratory studies that clarify the role alpha-4 beta-7 plays in HIV infection and offers a mechanism for how an antibody that blocks the receptor can help control HIV and SIV. Guzzo participated in a media briefing at CROI, joined by Fauci via video.

Soon after infection, SIV and HIV establish reservoirs in longed-lived CD4 T-cells, where viral genetic material can remain in an inactive state indefinitely. ART keeps viral replication under control, but once the drugs are stopped the virus reactivates and resumes its attack on immune cells. T-cells in the gut are especially vulnerable during early infection.

Guzzo and Fauci explained that the alpha-4 beta-7 integrin receptor on T-cells helps them "home" or migrate to gut tissue. Previous studies have shown that cells expressing alpha-4 beta-7 are highly susceptible to HIV infection, and that blocking alpha-4 beta-7 leads to reduced transmission of SIV and lower viral load in monkeys.

"To make a long story short, we found out almost by accident that, unbeknownst to us, the alpha-4 beta-7molecule on CD4 T-cell is a receptor for the HIV envelope," Fauci said.

In the study reported last October, among the 11 monkeys treated with alpha-4 beta-7 antibody infusions, 2 maintained viral suppression after stopping ART -- the longest for nearly 2 years of follow-up -- while 6 others experienced temporary viral rebound followed by re-suppression. In contrast, all 7 monkeys that received placebo infusions experienced high-level sustained SIV rebound.

"It was a striking finding, but we didn't know what the mechanism was -- we just knew that the result was very profound," Fauci recalled.

Guzzo explained that the gut lining contains epithelial cells that express adhesion moleculessuch as ICAM1 and MAdCAM1, whichwaitto capturecells that express the appropriate receptors. MAdCAM1, which is largely confined to the gut, looks for the alpha-4 beta-7 receptor, and thus attracts CD4 T-cells that carry this receptor.

The latest research shows that the same mechanism may attract SIV or HIV itself to the gut. As newly created virus particles burst out of a host cell, they capture part of the cell's membrane, and its alpha-4 beta-7 receptors become incorporated into the viral envelope.

Guzzo's team found that virus-associated alpha-4 beta-7 is functionally active, just as it is on the cell surface, and binds readily to MAdCAM1. They then created engineered virus particles with or without alpha-4 beta-7, and found that only virus that carried alpha-4 beta-7 was captured by MAdCAM1 and infected target cells in the laboratory.

Looking at HIV-1 from human patients, they found that circulating virus consistently incorporates alpha-4 beta-7 during both acute and chronic infection. Looking at monkeys experimentally infected with SIV, they found that alpha-4 beta-7 incorporation was highest during early infection.

Finally, they showed that HIV carrying alpha-4 beta-7 migrated to gut tissue in mice (which have compatible MAdCAM1), while HIV without alpha-4 beta-7did not. The same alpha-4 beta-7 antibody used in the monkey remission study potently blocked this gut homing, but a control antibody did not. And virus with and without alpha-4 beta-7 did not migrate differentially to lymph node and spleen tissue -- the effect was seen only in the gut.

Based on these findings, the researchers concluded, "Incorporation of alpha-4 beta-7into HIV-1 virions can promote tropism and retention of virus in gut tissues, which may impact transmission and pathogenesis."

While this research answers some questions, much remains to be learned. Fauci noted that in the monkey study the virus did not rebound for such a long time that the infused antibodies were no longer present, suggesting that the therapy "somehow induced some sort of immune response that has continued to suppress the virus long after the antibody to alpha-4 beta-7 was gone."

One hypothesis is that since alpha-4 beta-7 is expressed on both virus particles and susceptible cells, the antibody may capture both and present them to the immune system in a way that induces a long-lasting immunological response. The antibody therapy might "teach the immune system to recognize HIV in a way it doesn’t recognize it if it just sees HIV alone," Fauci said. "Our job now is to find out just what the nature of that immune response is."

A small clinical trial is now underway to test the antibody therapy in people with chronic HIV infection on suppressive ART (NCT02788175). They will receive vedolizumab infusions over 30 weeks, then will stop both their antiretrovirals and vedolizumab to see if viral rebound occurs. Fauci said that so far 12 people have received infusions, starting in late August. Preliminary results are expected by the end of 2017.